1. Field of the Invention
The present invention relates to an image pickup apparatus which utilizes a charge coupled solid-state image pickup device of a frame interline transfer system (FIT-CCD), and particularly relates to an image pickup apparatus having an electronic shutter function to perform picking up of an image.
2. Description of Background Art
A charge coupled solid-state image pickup device for use in such an image pickup apparatus has a structure as shown in FIG. 3
That is, a charge coupled solid-state image pickup device is constituted by a drain portion 1 for discharging unnecessary electric charges, a photo-detection portion 2 for photo-detecting an optical image of a subject, a storage portion 3 for temporarily holding signal charges for every picture element produced in the photo-detection portion 2, and a horizontal charge transfer line 4 for reading out the signal charges in the storage portion 3. The charge coupled solid-state image pickup device is formed by a semiconductor production technique.
Further describing the structures of the respective portions, first, in the photo-detection portion 2, a plurality of photo-diodes are arranged in a matrix in vertical and horizontal scanning directions V and H. For example, in the case of a primary color stripe filter, as shown in FIG. 3, color filters of red (R) are laminated on the respective surfaces of a group of photo-diodes arranged in the first column, color filters of green (G) are laminated on the respective surfaces of a group of photodiodes arranged in the second column, and color filters of blue (B) are laminated on the respective surfaces of a group of photodiodes arranged in the third column. These three columns, as a set, are arranged repeatedly in the horizontal scanning direction H. The photo-diodes correspond to the respective picture elements. Assuming that these photo-diodes are arranged in M rows (M is an even number) in the vertical scanning direction V, a photo-diode group in an odd numbered row numbered from the side of the drain portion 1 is regarded as a first field, and a photo-diode group in an even numbered row is regarded as a second field.
Vertical charge transfer lines 1.sub.1 to 1.sub.N are formed adjacently to photo-diode groups in the respective columns, and four-phase driving signals .phi.I1, .phi.I2, .phi.I3 and .phi.I4 are applied to a transfer electrode group (not shown) laminated on the upper surface of the vertical charge transfer lines so as to produce transfer elements to transfer signal charges in the vertical scanning direction V. Further, photo-shield layers are formed on the upper surfaces of all the vertical charge transfer lines 1.sub.1 to 1.sub.N to prevent light from striking the upper surfaces.
Further, transfer gates (for example, shown as a representative by TG in FIG. 3) are provided between respective photodiodes and charge transfer elements in vertical charge transfer lines adjacent thereto, for conducting and therefore transferring signal charges produced in the respective photo-diodes to the charge transfer elements in the vertical charge transfer lines. Then, gate electrodes for driving the transfer gates are formed integrally with the transfer electrodes in the vertical charge transfer lines, and the transfer gates are set in a conductive state by setting driving signals to a high voltage at a predetermined timing.
The drain portion 1 is constituted by a redetermined impurity layer formed so as to connect with the first ends of all the vertical charge transfer lines 1.sub.1 to 1.sub.N, for transferring unnecessary charges transferred through the vertical charge transfer lines, to a semiconductor substratum.
The storage portion 3 is constituted by a charge transfer line group provided continuously with the other ends of all the vertical charge transfer lines 1.sub.1 to 1.sub.N, and four-phase driving signals .phi.S1, .phi.S2, .phi.S3 and .phi.S4 are applied to a transfer electrode group (not shown) laminated on the upper surfaces of the vertical charge transfer lines so as to have a function to transfer signal charges from the photo-detection portion 2 in the vertical scanning direction V and hold the signal charges in a predetermined charge transfer element group by stopping the driving signals temporarily. It is therefore possible to produce a transfer element group to temporarily hold signal charges produced in photo-diodes of M/2 lines (that is, one field). Further, photo-shield layers are formed on the upper surfaces of all the vertical charge transfer lines to prevent light from striking the upper surfaces.
The horizontal charge transfer line 4 is connected with the ends of all the charge transfer line groups of the storage portion 3 so as to transfer signal charges in the horizontal scanning direction H synchronously with two-phase driving signals .phi.H1A and .phi.H2 applied to a transfer electrode group (not shown) formed on the upper surface of the horizontal charge transfer line 4. The signal charges transferred synchronously with the two-phase driving signals .phi.H1A and .phi.H2 are impedance-converted in a floating diffusion amplifier 5 synchronously with a reset signal .phi.RS and an output gate signal .phi.H1B, and supplied to an output terminal 6 as a color signal for every picture element.
FIG. 4 shows image pickup timing in an electronic still camera or the like having an electronic shutter function, and FIGS. 5(a) to 5(h) show the operations at respective points in time t1 to t8. Then, for the purpose of simplification of description, FIG. 5 shows read-out scanning about a group of picture elements of three columns and two rows of red (R), blue (B) and green (G) which are adjacent to one another, as representative.
Describing the operation with reference to FIGS. 4 and 5, first, VD in FIG. 4 assumes an "H" level every 1/60 second.
Assuming that a shutter release button of an electronic still camera is pushed at the point of time t1, a charge transfer mode for a first field is set, and synchronously with that, the four-phase driving signals .phi.S1, , .phi.S2, .phi.S3 and .phi.S4 are set to an "M" level (voltage for producing a transfer element in a vertical charge transfer line) and at the same time, only the driving signal I1 is set in a high voltage "HH", so that as shown in FIG. 5(a), a transfer gate corresponding to the first field is made conductive (that is, the potential level thereof becomes deeper than that of the photo-diodes), so that unnecessary charges in all the photo-diodes in the first field are transferred to a transfer element in an adjacent vertical charge transfer line. At the same time, the charge transfer operation of the horizontal charge transfer line 4 starts so as to discharge unnecessary charges in the horizontal charge transfer line 4 to the outside through the floating diffusion amplifier 5 within a predetermined period.
Next, the charge transfer mode for the first field is switched to that for a second field, and only the driving signal .phi.I3 is set to a high voltage "HH" at the point of time t2, so that as shown in FIG. 5(b), a transfer gate corresponding to the second field is made conductive so that unnecessary charges in all the photo-diodes in the second field are transferred to a transfer element in an adjacent vertical charge transfer line.
By these transfer operations at the points of time t1 and t2, unnecessary residual charges in all the photo-diodes are transferred to the vertical charge transfer lines.
Next, in a predetermined period in a vertical fly-back period, the vertical charge transfer lines of the photo-detection portion 2 and the charge transfer lines of the storage portion 3 transfer and issue unnecessary charges to the side of the drain portion 1 synchronously with the driving signals .phi.I1 to .phi.I4 and .phi.S1 to .phi.S4. FIG. 5(c) shows a certain point of time t3 in the above transfer operation.
Next, at a point of time t4 of completion of the discharge operation of all unnecessary charges, the charge transfer mode for the second field is switched to that for a first field again, the driving signal .phi.I1 is set to a high voltage "HH" in the same manner as at the point of time t1, so that as shown in FIG. 5(d), a transfer gate corresponding to the first field is made conductive, so that charges in all the photo-diodes in the first field are transferred to a transfer element in an adjacent vertical charge transfer line. That is, a period .tau.1 from the point of time of making the transfer gate non-conductive after the point of time t1 till the point of time of making the transfer gate conductive is an exposure time of a photo-diode group corresponding to the first field.
Next, by a high speed charge transfer operation in a predetermined period, signal charges in the vertical charge transfer lines are transferred to the charge transfer lines of the storage portion 3. FIG. 5(e) shows the operation at a certain point of time t5 in the transfer operation. When this transfer operation is completed, all the signal charges of the photo-diode group corresponding to the first field are held by the storage portion 3.
Next, the charge transfer mode for the first field is switched to that for a second field again, and the driving signal .phi.I3 is set in a high voltage "HH" at a point of time t6 after the lapsing of the period .tau.2 from the point of time t2, so that as shown in FIG. 5(f), a transfer gate corresponding to the second field is made conductive, so that charges in all the photo-diodes in the second field are transferred to a transfer element in an adjacent vertical charge transfer line. That is, the period .tau.2 from the point of time of making the transfer gate non-conductive after the point of time t2 till the point of time t6 of making the transfer gate conductive again is an exposure time of a photo-diode group corresponding to the second field.
Next, while the signal charges corresponding to the second field are left stopped in the vertical charge transfer lines, the signal charges corresponding to the first field in the storage portion 3 are vertical-charge-transferred to the horizontal charge transfer line 4, and at the same time, the horizontal charge transfer line 4 performs horizontal charge transfer every vertical charge transfer of one horizontal line, so as to output color signals corresponding to respective picture elements corresponding to the first field (in the period .DELTA..tau.1 in FIG. 4).
When read-out of the signal charges in the storage portion 3 is completed, signal charges in the vertical charge transfer lines of the photo-detection portion 2 (charges corresponding to the second field) are thereafter transferred to the storage portion 3. This state is shown in FIG. 5(g) corresponding to the point of time t7.
Next, when all the signal charges corresponding to the second field are transferred to the storage portion 3, the signal charges are vertical-charge-transferred to the horizontal charge transfer line 4, and at the same time, the horizontal charge transfer line 4 performs horizontal charge transfer every time vertical charge transfer of one horizontal line is carried out so as to output color signals corresponding to respective picture elements corresponding to the second field (in the period .DELTA..tau.2 in FIG. 4). This state is shown in FIG. 5(h).
As has been described, by making a reset operation synchronously with a shutter release button and transferring signal charges integrated in photo-diodes to vertical charge transfer lines corresponding to a shutter speed, it is possible to obtain an artificial frame electronic shutter function.
However, in such a conventional image pickup apparatus, there has been a problem that field and color flicker becomes conspicuous as the shutter speed is made higher.
That is, as shown in a timing chart in FIG. 4, since signal charges corresponding to a second field are read after signal charges corresponding to a first field are read, the signal charges corresponding to the second field are stopped in vertical charge transfer lines in a photo-detection portion temporarily, so that a smear produced in this stop period is mixed in the signal charges corresponding to the second field, and this smear component causes flicker. In FIG. 4, the smear component is mixed in the signal charges corresponding to the second field in a period .tau.1, and the influence of a smear to the signal charges corresponding to the second field becomes larger than that to the first charges corresponding to the first field.
It is thought that this smear component is caused by light incident through an opening portion of a photo-diode which is reflected by a silicon oxide film under a charge transfer line and which reaches a vertical charge transfer line. Other causes of smear may be high intensive light which produces unnecessary charges in a semiconductor substratum, or incident light penetrating a photo-shield film formed on the upper surface of a vertical charge transfer line, or the like.
Further, as shown in FIG. 6, because of different mixture rates of smear components in respective color signals of red (R), green (G) and blue (B), there has been a problem of not only field flicker, in the case of still picture reproduction, but also color flicker in the case of reproduction of a color picture.
That is, FIG. 6 shows the level difference (indicated by percent) between picture element signals read from picture elements corresponding to first and second fields, by shutter speed and colors of red (R), green (G) and blue (B) in the case of picking up an image by a conventional artificial frame electronic shutter function. As is apparent from FIG. 6, since incident light intensity is higher as the shutter speed is increased, a smear component in a picture element signal corresponding to the second field is more increased than that in a picture element signal corresponding to the first field, and if the difference reaches more than about 1 percent, flicker can be recognized easily by human eyes. Moreover, since there are different characteristics in regard to the shutter speed among the three primary colors of red (R), blue (B) and green (G), not only luminance-based field flicker but also color-based color flicker are produced.